Actually the Nazis operated U boats in the St Lawrence river, east of Quebec city, and attacked individual ships on the surface at night, using their deck guns, to conserve torpedoes.
This had the effect of drawing badly needed escort ships fom the convoys that were leaving from Halifax to cross the Atlantic. The St Lawrence was a hard place to do ASDIC scans, to try to locate U boats, due to the mixing of fresh and salt water. This created layers of reflective water, that decreased their effectiveness. A further problem was the current and fluctuating tidal bores, near the Gulf of St Lawrence opening. A number of RCN ships were based along the north coast of the Gaspe penninsula, to do convoy protection patrols.
A large number of mechant ships were sunk IN the St Lawrence, by U boats. Also included in that number were some RCN ships. So yes , the St Lawrence river and Gulf were very much a battleground in WW2.
Jim Bunting. Toronto.